A large number of drugs that exhibit potent antibiotic, antitumor and/or antiviral activity belong to the structural class of compounds known as C-glycosides, in which a carbohydrate moiety is attached to a carbon atom of a typically hydrophobic aglycon unit. Although their glycons are not particularly hydrophobic, C-nucleosides are the most representative of these C-glycosides both in their abundance and in their biological activities. Numerous C-glycosides are currently on the market as medicinal drugs. Therefore, development of an improved method for the synthesis of such compounds, especially their structural analogs that may possess enhanced pharmacological profiles, continues to be an area of intense commercial interest in the pharmaceutical and chemical industry (for reviews, see: Hacksell, U.; Daves, G. D., Jr. Prog. Med. Chem. 1985, 22, 1-65 and Daves, G. D., Jr. Acc. Chem. Res. 1990, 23, 201-206 both incorporated herewith by reference).
While several reactions that utilize glycal derivatives as glycosylating reagents have been reported employing various Lewis acids as catalysts for C-glycosylation, the harsh nature of these Lewis acids has prevented their application to the synthesis of the C-glycosylated derivatives of acid-labile substrates. These Lewis acids include boron trifluoride etherate (Dawe, R. D.; Fraser-Reid, B. J.C.S. Chem. Commun.1981, 1180-1181; Panek, J. S.; Sparks, M. A. J. Org. Chem. 1982, 47, 3805-3806; Sabol, J. S.; Cregge, R. J. Tetrahedron Lett. 1989, 30, 6271-6274), ethyldichloroaluminum, and trimethylsilyl trifluoromethanesulfonate (for the use of both of these Lewis acids, see: Herscovici, J.; Muleka, K.; Antonakis, K. Tetrahedron Lett. 1984, 25, 5653-5656). In addition, since most of these strong Lewis acids spontaneously react with air and moisture, the use of these Lewis acids presents serious problems in their handling, particularly under the large-scale, industrial setting. Another approach to C-glycosylation that employs a glycal derivative requires the use of expensive metal catalyst whose effects to human health could potentially be serious drawbacks (Hacksell, U.; Daves, G. D., Jr. J. Org. Chem. 1983, 48, 2870-2876).